Structure of the biliverdin cofactor in the Pfr state of bathy and prototypical phytochromes

Phytochromes act as photoswitches between the red- and far-red absorbing parent states of phytochromes (Pr and Pfr). Plant phytochromes display an additional thermal conversion route from the physiologically active Pfr to Pr. The same reaction pattern is found in prototypical biliverdin-binding bacteriophytochromes in contrast to the reverse thermal transformation in bathy bacteriophytochromes. However, the molecular origin of the different thermal stabilities of the Pfr states in prototypical and bathy bacteriophytochromes is not known. We analyzed the structures of the chromophore binding pockets in the Pfr states of various bathy and prototypical biliverdin-binding phytochromes using a combined spectroscopic-theoretical approach. For the Pfr state of the bathy phytochrome from Pseudomonas aeruginosa, the very good agreement between calculated and experimental Raman spectra of the biliverdin cofactor is in line with important conclusions of previous crystallographic analyses, particularly the ZZEssa configuration of the chromophore and its mode of covalent attachment to the protein. The highly homogeneous chromophore conformation seems to be a unique property of the Pfr states of bathy phytochromes. This is in sharp contrast to the Pfr states of prototypical phytochromes that display conformational equilibria between two sub-states exhibiting small structural differences at the terminal methine bridges A-B and C-D. These differences may mainly root in the interactions of the cofactor with the highly conserved Asp-194 that occur via its carboxylate function in bathy phytochromes. The weaker interactions via the carbonyl function in prototypical phytochromes may lead to a higher structural flexibility of the chromophore pocket opening a reaction channel for the thermal (ZZE → ZZZ) Pfr to Pr back-conversion.

Light-induced activation of bacterial phytochrome Agp1 monitored by static and time-resolved FTIR spectroscopy

Phytochromes, which regulate many biological processes in plants, bacteria, and fungi, can exist in two stable states, Pr and Pfr, that can be interconverted by light, via a number of intermediates such as meta-Rc. Herein we employ FTIR spectroscopy to study the Pr-to-Pfr conversion of the bacteriophytochrome Agp1 from Agrobacterium tumefaciens. Static FTIR Pfr/Pr and meta-Rc/Pr difference spectra are disentangled in terms of cofactor and protein structural changes. Guided by DFT calculations on cofactor models, the chromophore conformational changes can be grouped into structural adjustments of the cofactor-protein interactions localized in the C-D dipyrrole moiety, that is, the photoisomerisation site, and in the A-B dipyrrole moiety including the protein attachment site. Whereas changes at the C and D rings appear to be largely completed in the meta-Rc state, the structural changes in the A-B unit occur during the transition from meta-Rc to Pfr, concomitant with the main protein structural changes, as demonstrated by static and time-resolved FTIR difference spectroscopy. We employ this technique to monitor, for the first time, the dynamics of the photocycle of phytochrome on the millisecond timescale. By extending the studies to genetically engineered protein variants of Agp1, we further demonstrate that H250 and D197 as well as the PHY domain are essential for formation of the Pfr state. Based on the IR spectroscopic and available crystallographic data we discuss the role of critical amino acid residues for the protein-cofactor interactions during the photoinduced reaction cycle.

Sub-picosecond mid-infrared spectroscopy of phytochrome Agp1 from Agrobacterium tumefaciens

The photoinduced primary reaction of the biliverdin binding phytochrome Agp1 (Agp1-BV) from Agrobacterium tumefaciens was investigated by sub-picosecond time-resolved Vis pump-IR probe spectroscopy. Three time constants of tau(1)=0.7+/-0.05 ps, tau(2)=3.3+/-0.2 ps and tau(3)=33.3+/-1.5 ps could be isolated from the dynamics of structurally specific marker bands of the BV chromophore. These results together with those of accompanying sub-picosecond Vis pump-Vis probe spectroscopy allow the extension of the reaction scheme for the primary process by a vibrationally excited electronic ground state. The isomerization at the C15=C16 bond occurs within the lifetime of the excited electronic state. A quantum yield of 0.094 for the primary reaction is determined, suggesting that the quantum yield of formation of the P(fr) far-red-absorbing form is already established in the primary photoreaction of the P(r) (red-absorbing) form.